Nonaqueous electrolyte secondary battery

ABSTRACT

An electrode body of a secondary battery described herein includes: a core portion where electrode mixture layers of a plurality of electrode sheets are laminated; terminal connecting portions where respective current collector foil exposed portions are laminated, and a mixture layer non-facing portion where the electrode mixture layer faces the current collector foil exposed portion, the mixture layer non-facing portion being formed in a boundary between the terminal connecting portion and the core portion. In the secondary battery described herein, a short-circuit promoting portion having a predetermined depth (d) is formed in a separator provided between the electrode sheets in the mixture layer non-facing portion. Hereby, before a battery temperature rapidly increases to a high-temperature range due to occurrence of internal short-circuit between the electrode mixture layers, internal short-circuit is caused between the electrode mixture layer and the current collector foil exposed portion, so that charging can be stopped.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2019-175415 filed onSep. 26, 2019 including the specification, drawings and abstract isincorporated herein by reference in its entirety.

BACKGROUND 1. Technical Field

The present disclosure relates to a nonaqueous electrolyte secondarybattery.

2. Description of Related Art

In recent years, a nonaqueous electrolyte secondary battery such as alithium-ion secondary battery has been used appropriately for a portablepower supply for a personal computer, a portable terminal, and the like,and a vehicle drive power supply for an electric vehicle (EV), a hybridvehicle (HV), a plug-in hybrid vehicle (PHV), and the like.

In some cases, when a large current flows through the nonaqueouselectrolyte secondary battery at the time of charging and an overchargestate is caused, a battery temperature might increase to ahigh-temperature range of 200° C. or more. As one technology to securehigh safety by preventing such a temperature increase to thehigh-temperature range in the overcharge, there is a technology toprovide a shutdown function to a separator provided between a positiveelectrode and a negative electrode. More specifically, microporesthrough which charge carriers (e.g., lithium ions) are passed are formedin the separator of the nonaqueous electrolyte secondary battery. Theseparator having the shutdown function is configured to close themicropores by melting due to a temperature increase at an early stage ofovercharge, so that charging can be stopped before the temperatureincrease to the high-temperature range starts. One example of theseparator having such a shutdown function is described in JapaneseUnexamined Patent Application Publication No. 2006-331922 (JP2006-331922 A).

SUMMARY

In the meantime, in the field of the nonaqueous electrolyte secondarybattery in recent years, the development of a technology that can moresurely prevent the temperature increase to the high-temperature range incase of overcharge has been demanded due to an increasing need forsafety. For example, in a case where an excessive current is supplied atan early stage of overcharge and a temperature largely increases,melting of the separator might proceed rapidly. In this case, internalshort-circuit might occur due to contact between the positive electrodeand the negative electrode before charging is stopped (the shutdownfunction works) by blocking charge carriers, so that the batterytemperature might rapidly increase to the high-temperature range.

The present disclosure provides a nonaqueous electrolyte secondarybattery that can prevent a rapid increase in a battery temperature to ahigh-temperature range when an excessive current is supplied duringcharging.

In order to achieve the above object, the present disclosure provides anonaqueous electrolyte secondary battery (hereinafter also just referredto as a “secondary battery”) having the following configuration.

The secondary battery described herein includes: an electrode body inwhich a plurality of electrode sheets is laminated via a separator; anda pair of electrode terminals electrically connected to the electrodebody. Each of the electrode sheets is a sheet-shaped electrode in whichan electrode mixture layer is formed on a surface of a current collectorfoil, the each of the electrode sheets including a current collectorfoil exposed portion in which the current collector foil is exposed, thecurrent collector foil exposed portion being formed in a correspondingone of side edge portions, in the width direction, of the each of theelectrode sheets. The electrode body includes: a core portion where theelectrode mixture layers of the electrode sheets are laminated in acentral part of the electrode body in the width direction; terminalconnecting portions to which the electrode terminals are connected,respectively, the terminal connecting portions being formed in both sideedge portions of the electrode body in the width direction such thatrespective current collector foil exposed portions are laminated; and amixture layer non-facing portion formed in at least one of boundaries ofthe core portion with the terminal connecting portions, the mixturelayer non-facing portion being configured such that the electrodemixture layer faces the current collector foil exposed portion. Ashort-circuit promoting portion constituted by a recessed portion havinga depth of 30% or more of a thickness of the separator is formed in theseparator provided between the electrode sheets in the mixture layernon-facing portion.

In the secondary battery described herein, the recessed portion (theshort-circuit promoting portion) having a predetermined depth is formedin the separator. When the separator having such a structure is heated,the separator starts to melt from a position where the short-circuitpromoting portion is formed. On this account, by adjusting a formationposition of the short-circuit promoting portion, a position whereinternal short-circuit is to occur when a temperature increases due tosupply of an excessive current can be controlled. In the secondarybattery described herein, the short-circuit promoting portion is formedin the mixture layer non-facing portion where the electrode mixturelayer faces the current collector foil exposed portion. According to theexamination of the inventors of the present disclosure, a suddentemperature increase to a high-temperature range occurs when internalshort-circuit occurs between the electrode mixture layers in the coreportion of the electrode body. Even when internal short-circuit occursbetween the electrode mixture layer and the current collector foilexposed portion (the current collector foil) in the mixture layernon-facing portion, charging stops without a sudden temperatureincrease. That is, with the secondary battery described herein, theformation position of the short-circuit promoting portion in theseparator is adjusted so that internal short-circuit is caused betweenthe electrode mixture layer and the current collector foil when atemperature increases due to supply of an excessive current. This makesit possible to prevent a battery temperature from rapidly increasing toa high-temperature range.

The short-circuit promoting portion may be an elongated groove extendingalong a side edge of the core portion.

Hereby, heat generated in the core portion at the time when a largetemperature increase occurs in the core portion of the electrode body isefficiently transmitted to the short-circuit promoting portion. Thismakes it possible to appropriately cause internal short-circuit in themixture layer non-facing portion.

Joining portions where the terminal connecting portions are joined tothe electrode terminals, respectively, may be formed in respectiveconnection parts of the terminal connecting portions with the electrodeterminals. A total area of the short-circuit promoting portion may be25% or more of an area of a joining portion closer to the short-circuitpromoting portion out of the joining portions.

In a general secondary battery, at the time when an excessive current issupplied, the joining portion between the terminal connecting portionand the electrode terminal may suddenly generate heat in some cases. Inthis aspect, the area of the short-circuit promoting portion is definedin consideration of heat generation in the joining portion. Thisaccordingly makes it possible to more appropriately cause internalshort-circuit in the mixture layer non-facing portion.

The short-circuit promoting portion may be formed in a separator placedon an outermost side in the thickness direction of the electrode body.

This makes it possible to easily form the short-circuit promotingportion, thereby making it possible to restrain a decrease in productionefficiency to be cause by providing a step of forming the short-circuitpromoting portion.

The short-circuit promoting portion may be formed in a separator placedon an inner side in the thickness direction of the electrode body.

Inside an electrode body in a general secondary battery, heat is easilygenerated, but the heat is hard to dissipate. Accordingly, the inside ofthe electrode body tends to easily increase in temperature. Inconsideration of this point, the short-circuit promoting portion isformed in the separator placed inside the electrode body. This makes itpossible to appropriately cause internal short-circuit in the mixturelayer non-facing portion.

BRIEF DESCRIPTION OF THE DRAWINGS

Features, advantages, and technical and industrial significance ofexemplary embodiments of the disclosure will be described below withreference to the accompanying drawings, in which like numerals denotelike elements, and wherein:

FIG. 1 is a perspective view schematically illustrating a nonaqueouselectrolyte secondary battery according to one embodiment of the presentdisclosure;

FIG. 2 is a front view schematically illustrating an internal structureof the nonaqueous electrolyte secondary battery according to oneembodiment of the present disclosure;

FIG. 3 is a perspective view schematically illustrating a woundelectrode body in one embodiment of the present disclosure;

FIG. 4 is a view schematically illustrating a section of the woundelectrode body along the width direction in one embodiment of thepresent disclosure;

FIG. 5 is a side view schematically illustrating a wound electrode bodyin another embodiment of the present disclosure;

FIG. 6 is a perspective view schematically illustrating a configurationof a stacked electrode body in another embodiment of the presentdisclosure; and

FIG. 7 is an explanatory view schematically illustrating each memberconstituting the stacked electrode body in another embodiment of thepresent disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS

The following describes a lithium-ion secondary battery as an example ofa nonaqueous electrolyte secondary battery according to one embodimentof the present disclosure with reference to the drawings. In thefollowing drawings, the same reference sign is assigned to members orparts that yield the same effect. Note that a dimensional relationship(length, width, thickness, and the like) in each drawing does not showan actual dimensional relationship. Further, a matter that is notparticularly mentioned in the present specification but is necessary forexecution of the present disclosure (e.g., an electrolyte composition, amanufacturing method, and so on) can be understood as a design matter ofa person skilled in the art based on conventional techniques in the art.Note that a structure of the nonaqueous electrolyte secondary batterydescribed herein is not limited to a lithium-ion secondary battery andis applicable to various secondary batteries (e.g., a nickel-metalhydride battery).

1. First Embodiment

FIG. 1 is a perspective view schematically illustrating a nonaqueouselectrolyte secondary battery according to the present embodiment. FIG.2 is a front view schematically illustrating an internal structure ofthe nonaqueous electrolyte secondary battery according to the presentembodiment. FIG. 3 is a perspective view schematically illustrating awound electrode body in the present embodiment. Further, FIG. 4 is aview schematically illustrating a section of the wound electrode bodyalong the width direction in the present embodiment. Further, in eachdrawing, a reference sign X indicates “the width direction (of thenonaqueous electrolyte secondary battery),” a reference sign Y indicates“the thickness direction (of the nonaqueous electrolyte secondarybattery),” and a reference sign Z indicates “the height direction (ofthe nonaqueous electrolyte secondary battery).” Note that thesedirections are defined for purpose of this description and do not intendto limit a direction to provide the nonaqueous electrolyte secondarybattery described herein.

(1) Case

As illustrated in FIG. 1, a nonaqueous electrolyte secondary battery 1according to the present embodiment includes a flat square case 10. Thecase 10 includes a square case main body 12 having an upside opening(not shown), and a cover 14 that closes the upside opening. It ispreferable that the case 10 be mainly made of a metallic material havinga light weight and a high strength, e.g., aluminum alloy and the like.As illustrated in FIG. 2, an electrode body 20 is accommodated insidethe case 10. Further, although not illustrated herein, a nonaqueouselectrolyte (typically, a nonaqueous electrolyte solution) is alsoaccommodated inside the case 10 in addition to the electrode body 20. Asthe nonaqueous electrolyte, any nonaqueous electrolyte that is used in ageneral lithium-ion secondary battery can be used without any particularlimit. The nonaqueous electrolyte does not characterize the presentdisclosure and therefore is not described herein.

Further, the secondary battery 1 according to the present embodimentincludes a pair of electrode terminals electrically connected to theelectrode body 20. The connection between the electrode body 20 and theelectrode terminals will be described later more specifically. However,in the present specification, an electrode terminal connected to apositive side of the electrode body 20 is referred to as a “positiveterminal 16,” and an electrode terminal connected to a negative side isreferred to as a “negative terminal 18.” The positive terminal 16 andthe negative terminal 18 are attached to the cover 14 of the case 10.

(2) Electrode Body

As illustrated in FIG. 3, the electrode body 20 is formed by laminatinga plurality of electrode sheets 50, 60 via separators 70. The electrodebody 20 used in the present embodiment is a flat-shaped wound electrodebody. The flat-shaped wound electrode body 20 is formed such that alaminated body in which a pair of positive and negative electrode sheets50, 60 is laminated via two separators 70 is formed, and a wound bodyobtained by winding the laminated body in a laminated manner isflattened by pressing.

Note that means for forming the flat-shaped wound electrode body is notlimited to this, and means for winding the separators and the electrodesheets around a rectangular plate-shaped core in a laminated manner canbe employed, for example.

(3) Electrode

The electrode sheets 50, 60 are sheet-shaped electrodes in whichelectrode mixture layers 54, 64 are formed on respective surfaces ofcurrent collector foils 52, 62. Further, current collector foil exposedportions 52 a, 62 a 6 are formed in their corresponding side edgeportions, in the width direction X, of the electrode sheets 50, 60. Inthe current collector foil exposed portions 52 a, 62 a, the electrodemixture layers 54, 64 are not formed and the current collector foils 52,62 are exposed. Note that, in the present specification, the electrodesheet 50 on the positive side is referred to as the “positive electrodesheet 50,” and the electrode sheet 60 on the negative side is referredto as the “negative electrode sheet 60.” The following describes thepositive electrode sheet 50 and the negative electrode sheet 60 morespecifically.

(a) Positive Electrode Sheet

The positive electrode sheet 50 includes a positive-electrode currentcollector foil 52, and positive-electrode mixture layers 54 formed onboth surfaces of the positive-electrode current collector foil 52. In afirst side edge portion, in the width direction X, of the positiveelectrode sheet 50, a positive-electrode exposed portion 52 a in whichthe positive-electrode mixture layer 54 is not formed and thepositive-electrode current collector foil 52 is exposed is formed. Anymaterial used for a positive-electrode current collector foil in thistype of secondary battery can be used for the positive-electrode currentcollector foil 52 without any particular limit. Typically, it ispreferable that the positive-electrode current collector foil 52 be madeof metal that is cheap and has good conductivity, e.g., metals such asaluminum, nickel, titanium, and stainless steel, alloys including thosemetals, and so on.

The positive-electrode mixture layer 54 includes a positive-electrodeactive material. Examples of such a positive-electrode active materialinclude a composite oxide having a layer structure, represented byLiCoO₂, LiNiO₂, LiNi_(x)Co_(y)Mn_((1−x−y))O₂ (here, 0 <x <1, 0<y<1,0<x+y<1), and so on. Further, examples of the positive-electrode activematerial include a composite oxide having a spinel structure,represented by Li₂NiMn₃O₈, LiMn₂O₄, Li_(1+x)Mn_(2−y)M_(y)O₄ (here, M isnot present or one or more metal elements selected from Al, Mg, Co, Fe,Ni, and Zn, 0≤x<1, 0≤y<2), a composite compound having an olivinestructure such as LiFePO₄, and so on. Note that the positive-electrodeactive material does not limit the technology described herein, andvarious compounds conventionally used in this type of secondary batterycan be used. The positive-electrode active material will be omitted morespecifically.

Note that a given component other than the positive-electrode activematerial may be added to the positive-electrode mixture layer 54.Examples of the given component include a conductive material, a binder,and the like, for example. As the conductive material, a carbon materialsuch as acetylene black (AB), graphite, or carbon nanotube can be usedappropriately. Further, as the binder, a fluorine-based binder such aspolyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE), arubber-based binder such as styrene butadiene rubber (SBR), or the likecan be used.

(b) Negative Electrode Sheet

The negative electrode sheet 60 includes a negative-electrode currentcollector foil 62, and negative-electrode mixture layers 64 formed onboth surfaces of the negative-electrode current collector foil 62.Similarly to the positive electrode sheet 50, the negative electrodesheet 60 is also provided with a current collector foil exposed portion.More specifically, in a second side edge portion, in the width directionX, of the negative electrode sheet 60, a negative-electrode exposedportion 62 a in which the negative-electrode mixture layer 64 is notformed and the negative-electrode current collector foil 62 is exposedis formed. Any material used for a negative-electrode current collectorfoil in this type of secondary battery can be used for thenegative-electrode current collector foil 62 without any particularlimit. Typically, the negative-electrode current collector foil 62 canbe made of metal that is cheap and has good conductivity, e.g., copperor alloy mainly made of copper.

Further, as illustrated in FIG. 3, in the secondary battery according tothe present embodiment, in order to restrain precipitation of metal Lion the negative side by increasing the capacity of thenegative-electrode mixture layer 64, the negative-electrode mixturelayer 64 having a width w2 longer than a width w1 of thepositive-electrode mixture layer 54 is formed. The negative-electrodemixture layer 64 includes a negative-electrode active material. Examplesof such a negative-electrode active material include carbon materialssuch as graphite, meso-carbon microbeads, and carbon black (acetyleneblack, ketjen black, and the like). Note that the negative-electrodeactive material does not limit the technology described herein, andvarious compounds conventionally used in this type of secondary batterycan be used. The negative-electrode active material will be omitted morespecifically.

Further, a given component other than the negative-electrode activematerial may be added to the negative-electrode mixture layer 64. Forexample, a thickener, a binder, or the like can be added to thenegative-electrode mixture layer 64. As the thickener, carboxymethylcellulose (CMC) or the like can be used. Further, as the binder, afluorine-based binder such as PVDF or PTFE, a rubber-based binder suchas SBR, or the like can be used appropriately, similarly to thepositive-electrode mixture layer 54.

(4) Separator

The separator 70 is an insulating sheet-shaped member provided betweenthe electrode sheets 50, 60. In the electrode body 20 in the presentembodiment, two separators 70 having a width w3 longer than the width w2of the negative-electrode mixture layer 64 are used. One of theseparators 70 is placed on a back face of the positive electrode sheet50, and the other one of the separators 70 is placed on a back face ofthe negative electrode sheet 60. As such, when a laminated body in whichsheet-shaped members are laminated in order of the separator 70, thenegative electrode sheet 60, the separator 70, and the positiveelectrode sheet 50 is wound, the wound electrode body 20 in which thepositive electrode sheet 50 and the negative electrode sheet 60 arewound via the two separators 70 in a laminated manner is formed.

Further, a plurality of micropores through which charge carriers (e.g.,lithium ions) are passed is formed in the separator 70. This makes itpossible to move the charge carriers between the positive electrodesheet 50 and the negative electrode sheet 60 while short-circuit betweenthe positive electrode sheet 50 and the negative electrode sheet 60 isprevented. As a material for the separator 70, any material used for ageneral secondary battery can be used without any particular limit.Examples of the material for the separator 70 include resin materialssuch as polyethylene (PE), polypropylene (PP), polyester, cellulose, andpolyamide. Among them, a resin sheet made of polyolefin resin such as PEor PP melts at a temperature from about 80° C. to 140° C. (typically,90° C. to 120° C., preferably 100° C. to 110° C., and, for example, 105°C.), so that a shutdown function to close the micropores can beachieved. Further, the polyolefin resin described above is alsopreferable from the viewpoint of causing melting of the separator 70from a short-circuit promoting portion 72 (described later)appropriately. Note that the separator 70 may be a single-layeredstructure constituted by a single material or may be a structure inwhich two or more types of resin sheets made of different materials orhaving different properties (e.g., an average thickness, a porosity, orthe like) are laminated (e.g., a three-layer structure in which a PPlayer is laminated on each surface of a PE layer).

The short-circuit promoting portion 72 is formed in the separator 70used in the present embodiment. As illustrated in FIG. 4, theshort-circuit promoting portion 72 is a recessed portion having a depthd that is 30% or more of a thickness t of the separator 70. When theseparator 70 having such a short-circuit promoting portion 72 is heated,the separator 70 starts to melt from a thin position where theshort-circuit promoting portion 72 is formed. On this account, byadjusting a formation position of the short-circuit promoting portion 72in the separator 70, a position where internal short-circuit occursbetween the positive electrode sheet 50 and the negative electrode sheet60 when a temperature increases due to an excessive charge current canbe controlled. Note that, from the viewpoint of more surely causinginternal short-circuit at an intended position, the depth d of theshort-circuit promoting portion 72 is preferably 35% or more of thethickness t of the separator 70, more preferably 40% or more, furtherpreferably 45% or more, and particularly preferably 50% or more. In themeantime, from the viewpoint of causing intended internal short-circuit,the upper limit of the depth d of the short-circuit promoting portion 72is not limited particularly and may be 90% or less of the thickness t ofthe separator 70. Note that, in consideration of the strength of theseparator 70, the upper limit of the depth d of the short-circuitpromoting portion 72 is preferably 80% or less of the thickness t of theseparator 70, more preferably 75% or less, further preferably 70% orless, and particularly preferably 65% or less.

In the secondary battery according to the present embodiment, theshort-circuit promoting portion 72 is formed in the separator providedbetween the electrode sheets, in a region called a mixture layernon-facing portion where an electrode mixture layer faces a currentcollector foil exposed portion. Accordingly, at the time when anexcessive current is supplied during charging and a temperature largelyincreases, internal short-circuit is intentionally caused between theelectrode mixture layer and the current collector foil exposed portion(the current collector foil), so that charging can be stopped before arapid temperature increase to a high-temperature range occurs. Thefollowing describes details of the formation position of theshort-circuit promoting portion 72 in the secondary battery 1 accordingto the present embodiment, together with a detailed structure of thewound electrode body 20.

First, as illustrated in FIG. 3, in the present embodiment, at the timewhen the wound electrode body 20 is manufactured, the positive electrodesheet 50 and the negative electrode sheet 60 are laminated such that thepositive-electrode exposed portion 52 a protrudes from a first side inthe width direction X and the negative-electrode exposed portion 62 aprotrudes from a second side in the width direction X. In the woundelectrode body 20 obtained by winding the laminated body, three types ofregions, i.e., a core portion 22, terminal connecting portions 24, 26,and mixture layer non-facing portions 27, 28, are formed.

The core portion 22 is a region formed in a central part, in the widthdirection X, of the wound electrode body 20. In the core portion 22, theelectrode mixture layers 54, 64 of the electrode sheets 50, 60 arelaminated. In the core portion 22 in the present embodiment, thepositive-electrode mixture layer 54 and the negative-electrode mixturelayer 64 are wound in a laminated manner so as to face each other viathe separator 70. In the secondary battery 1 according to the presentembodiment, charge and discharge reactions by the movement of the chargecarriers mainly occur in the core portion 22.

Further, the terminal connecting portions 24, 26 are respective regionsformed in both side edge portions, in the width direction X, of thewound electrode body 20. The current collector foil exposed portions 52a, 62 a are laminated in the terminal connecting portions 24, 26,respectively, and the electrode terminals 16, 18 (see FIG. 2) areconnected to the terminal connecting portions 24, 26, respectively. Inthe present embodiment, the terminal connecting portion (apositive-electrode connecting portion) 24 on the positive side where thepositive-electrode exposed portion 52 a is wound in a laminated manneris formed in the first side edge portion in the width direction X, andthe terminal connecting portion (a negative-electrode connectingportion) 26 on the negative side where the negative-electrode exposedportion 62 a is wound in a laminated manner is formed in the second sideedge portion in the width direction X. The positive terminal 16 isconnected to the positive-electrode connecting portion 24, and thenegative terminal 18 is connected to the negative-electrode connectingportion 26 (see FIG. 2). The terminal connecting portions 24, 26 areconnected to the electrode terminals 16, 18, respectively, by use ofultrasonic welding, laser beam welding, resistance welding, or the like,so that joining portions 42, 44 are formed in respective connectionportions.

The mixture layer non-facing portions 27, 28 are regions each formed ina boundary between the core portion 22 and a corresponding one of theterminal connecting portions 24, 26 such that the electrode mixturelayers 54, 64 do not face each other. More specifically, as illustratedin FIG. 3, in the present embodiment, the width w2 of thenegative-electrode mixture layer 64 is set to be longer than the widthw1 of the positive-electrode mixture layer 54. Accordingly, in theboundary between the core portion 22 in the central part of the woundelectrode body 20 and each of the terminal connecting portions 24, 26provided in both side edge portions of the wound electrode body 20, acorresponding one of the mixture layer non-facing portions 27, 28 wherethe positive-electrode mixture layer 54 is not provided and thenegative-electrode mixture layer 64 is provided is formed. Morespecifically, in the boundary between the positive-electrode connectingportion 24 and the core portion 22, the mixture layer non-facing portion27 where the negative-electrode mixture layer 64 and thepositive-electrode exposed portion 52 a (the positive-electrode currentcollector foil 52) are wound in a laminated manner via the separators 70is formed. In the meantime, in the boundary between thenegative-electrode connecting portion 26 and the core portion 22, themixture layer non-facing portion 28 where the separators 70 and thenegative-electrode mixture layer 64 are wound in a laminated manner isformed.

As illustrated in FIGS. 3 and 4, in the secondary battery 1 of thepresent embodiment, the short-circuit promoting portion 72 is formed inthe separator 70 in the mixture layer non-facing portion 27 where theelectrode mixture layer (the negative-electrode mixture layer 64) facesthe current collector foil exposed portion (the positive-electrodeexposed portion 52 a). Hereby, at the time when the temperatureincreases due to an excessive charge current, the separator 70 placed inthe mixture layer non-facing portion 27 melts earlier than otherregions, so that internal short-circuit occurs between thenegative-electrode mixture layer 64 and the positive-electrode exposedportion 52 a. When internal short-circuit occurs between the electrodemixture layer and the current collector foil as such, charging isstopped. However, it is found by the inventors that the degree of thetemperature increase at this time is lower than the degree of atemperature increase in internal short-circuit between the electrodemixture layers to be caused in the core portion. Accordingly, in thepresent embodiment, the formation position of the short-circuitpromoting portion 72 is adjusted such that, at the time when anexcessive charge current is supplied and the temperature increases,charging is stopped by causing internal short-circuit between theelectrode mixture layer 64 and the current collector foil 52 beforeoccurrence of internal short-circuit in the core portion 22 that causesa sudden temperature increase to a high-temperature range. Accordingly,with the secondary battery 1 of the present embodiment, it is possibleto prevent a rapid increase in a battery temperature to thehigh-temperature range when an excessive current is supplied duringcharging, thereby making it possible to secure safety higher thanbefore.

Note that the short-circuit promoting portion 72 in the presentembodiment is an elongated groove extending along a side edge of thecore portion 22 (see FIG. 3). The short-circuit promoting portion 72having such an elongated groove shape is formed in the separator 70 overthe whole circumference of the wound electrode body 20. Theshort-circuit promoting portion 72 having such an elongated groove shapecan be formed by placing a scriber on a transfer line of the separator70 at the time when the wound electrode body 20 is formed. By formingthe short-circuit promoting portion 72 along the core portion 22, largeheat generated in the core portion 22 can be efficiently transmitted tothe short-circuit promoting portion 72. This makes it possible toappropriately melt the separator 70 in the mixture layer non-facingportion 27, thereby making it possible to prevent the occurrence ofinternal short-circuit in the core portion 22.

Further, another example of a heat generation part at the time when anexcessive charge current is supplied is the joining portions 42, 44 (seeFIG. 2) between the terminal connecting portions 24, 26 and theelectrode terminals 16, 18. In consideration of heat generation in thejoining portions 42, 44, it is preferable that a total area of theshort-circuit promoting portion 72 in a front view be 25% or more of anarea of a joining portion closer to the short-circuit promoting portion72 (herein, the joining portion 42 between the positive-electrodeconnecting portion 24 and the positive terminal 16). Further, from theviewpoint of more appropriately causing internal short-circuit in themixture layer non-facing portion 27, the total area of the short-circuitpromoting portion 72 is more preferably 30% or more of the area of thejoining portion 42, further preferably 35% or more, and particularlypreferably 40% or more. Further, the upper limit of the total area ofthe short-circuit promoting portion 72 is not limited particularly andmay be 90% or less of the area of the joining portion 42. Note that, inconsideration of the strength of the separator 70, the upper limit ofthe total area of the short-circuit promoting portion 72 is preferably80% or less of the area of the joining portion 42, more preferably 70%or less, further preferably 60% or less, and particularly preferably 50%or less. Note that the “total area of the short-circuit promotingportion” in the present specification indicates a sum of the area of allshort-circuit promoting portions formed on a separator surface in afront view and can be measured by examining respective surfaces of theseparators used herein by disassembling the electrode body.

2. Other Embodiments

The above description deals with the nonaqueous electrolyte secondarybattery according to one embodiment of the present disclosure. Note thatthe above embodiment is not intended to limit the present disclosure andcan be modified variously. The following describes nonaqueouselectrolyte secondary batteries according to other embodiments of thepresent disclosure.

(1) Shape of Short-circuit Promoting Portion

In the above embodiment, the short-circuit promoting portion 72 havingan elongated groove shape along the side edge of the core portion 22 isformed in the separator 70. However, the shape of the short-circuitpromoting portion in a front view is not limited particularly, providedthat the short-circuit promoting portion is a recessed portion having adepth that is 30% or more of the thickness of the separator. Forexample, a plurality of dot-shaped recessed portions (short-circuitpromoting portions) may be formed in the separator in the mixture layernon-facing portion. Further, a plurality of groove-shaped short-circuitpromoting portions along the width direction of the electrode body maybe formed in the separator in the mixture layer non-facing portion. Evenin these cases, at the time when the temperature largely increases,melting of the separator can be started from the mixture layernon-facing portion, and internal short-circuit can be caused between theelectrode mixture layer and the current collector foil.

Further, as illustrated in FIG. 4, in the above embodiment, theshort-circuit promoting portion 72 is formed on one surface of theseparator 70. However, the short-circuit promoting portions may beformed on both surfaces of the separator. Note that, in a case where theshort-circuit promoting portions are formed on both surfaces of theseparator, when the sum of depths of two short-circuit promotingportions is 30% or more of the thickness of the separator 70, theseparator can be melted from the position where the short-circuitpromoting portions are formed.

(2) Formation Position of Short-circuit Promoting Portion inCircumferential Direction

In the above embodiment, the short-circuit promoting portion 72 having agroove shape along the side edge of the core portion 22 is formed overthe whole circumference of the wound electrode body 20. However, it isnot necessary for the short-circuit promoting portion to be formed overthe whole circumference of the wound electrode body, and theshort-circuit promoting portion may be formed only at a specificposition in the circumferential direction. In this case, the strength ofthe separator can be secured sufficiently. For example, as illustratedin FIG. 5, the flat-shaped wound electrode body 20 includes a pair offlat portions 20 a facing each other in a side view, and a pair of Rportions 20 b formed in both ends of the wound electrode body 20 in theheight direction Z such that the electrode sheet curves. In theflat-shaped wound electrode body 20, the short-circuit promoting portionmay be formed only in the separators placed in the flat portions 20 a ormay be formed only in the separators placed in the R portions 20 b. Notethat the general flat-shaped wound electrode body 20 has such a tendencythat heat generated in the R portions 20 b is hard to dissipate. Inconsideration of this point, it is preferable that the short-circuitpromoting portion be formed in the separators placed in the R portions20 b.

Further, the short-circuit promoting portion may be formed only at aspecific position in the electrode body 20 in the thickness direction Y.For example, the short-circuit promoting portion can be easily formed inthe separator placed on the outermost side in the thickness direction Y.Accordingly, a decrease in production efficiency to be caused byproviding a step of forming the short-circuit promoting portion can berestrained. In the meantime, in a general secondary battery, the insideof the electrode body easily generates heat, but the heat is hard todissipate. Accordingly, the inside of the electrode body tends to easilyincrease in temperature. In consideration of this point, it ispreferable that the short-circuit promoting portion be formed in theseparator placed inside the electrode body 20 in the thickness directionY.

(3) Formation Position of Short-circuit Promoting Portion in Front View

As illustrated in FIG. 3, in the above embodiment, the width w2 of thenegative-electrode mixture layer 64 is longer than the width w1 of thepositive-electrode mixture layer 54. Accordingly, the mixture layernon-facing portion 27 where the negative-electrode mixture layer 64faces the positive-electrode exposed portion 52 a is formed between thepositive-electrode connecting portion 24 and the core portion 22. Theshort-circuit promoting portion 72 is formed in the separator 70 in themixture layer non-facing portion 27 between the positive-electrodeconnecting portion 24 and the core portion 22.

However, the mixture layer non-facing portion where the short-circuitpromoting portion is formed is not limited to the mixture layernon-facing portion where the negative-electrode mixture layer faces thepositive-electrode exposed portion. For example, by adjusting anapplication width of each of the positive-electrode andnegative-electrode mixture layers, respective laminated positions of thepositive electrode sheet and the negative electrode sheet, and the like,a mixture layer non-facing portion where the positive-electrode mixturelayer faces the negative-electrode exposed portion can be formed. Inthis case, the short-circuit promoting portion may be formed in theseparator in the mixture layer non-facing portion where thepositive-electrode mixture layer faces the negative-electrode exposedportion. Even in this case, at the time when the temperature largelyincreases, internal short-circuit is intentionally caused between theelectrode mixture layer and the current collector foil, so that a suddentemperature increase to the high-temperature range due to internalshort-circuit between the electrode mixture layers can be prevented.Further, the mixture layer non-facing portion does not need to be formedin the boundary between each of the terminal connecting portions and thecore portion. For example, in a case where the mixture layer non-facingportion where the electrode mixture layer faces the current collectorfoil exposed portion is formed in the boundary between one of theterminal connecting portions and the core portion, when theshort-circuit promoting portion is formed in the separator in thismixture layer non-facing portion, it is possible to prevent a suddentemperature increase to a high-temperature range due to internalshort-circuit between the electrode mixture layers. In addition, in acase where the electrode mixture layer faces the current collector foilexposed portion in both mixture layer non-facing portions, theshort-circuit promoting portions may be formed in the separators inthose mixture layer non-facing portions.

Note that the width of the positive-electrode mixture layer may be thesame as the width of the negative-electrode mixture layer. Even in thiscase, by adjusting respective laminated positions of the positiveelectrode sheet and the negative electrode sheet, it is possible to formthe mixture layer non-facing portions.

(4) Formation of HRL Layer

Further, an HRL layer (Heat Resistant Layer) may be formed on thesurface of the separator. The HRL layer is a layer including inorganicparticles (e.g., alumina particles or the like) having high heatresistance. When such an HRL layer is formed, it is possible to restrainmelting and contraction of the separator due to a temperature increase,thereby making it possible to more appropriately prevent internalshort-circuit between the electrode mixture layers in the core portion.Note that, in a case where the HRL-layer applied separator is used, theshort-circuit promoting portion may be formed in either of a surface ofthe separator on a side where the HRL layer is not formed and a surfaceof the separator on a side where the HRL layer is formed. In eithercase, it is possible to appropriately start melting of the separatorfrom a position where the short-circuit promoting portion is formed.Note that, from the viewpoint of facilitating formation of theshort-circuit promoting portion, it is preferable that the short-circuitpromoting portion be formed on the surface of the separator on the sidewhere the HRL layer is not formed. In the meantime, from the viewpointof facilitating melting from the short-circuit promoting portion, it ispreferable that the short-circuit promoting portion be formed on thesurface of the separator on the side where the HRL layer is formed.

(5) Structure of Electrode Body

The above embodiment is targeted for the secondary battery including thewound electrode body 20 as illustrated in FIG. 3. However, the electrodebody can employ various structures well known in the art as long as theeffect of the technology described herein is not obstructed. In otherwords, the electrode body in the technology described herein is notlimited to the wound electrode body in which the electrode sheets andthe separators are wound in a laminated manner, provided that theelectrode body is formed by laminating a plurality of electrode sheetsvia separators and includes a mixture layer non-facing portion where anelectrode mixture layer faces a current collector foil exposed portion.Another example of such an electrode body includes a stacked electrodebody in which a plurality of electrode sheets is stacked via separators.FIG. 6 is a perspective view schematically illustrating a configurationof the stacked electrode body in another embodiment, and FIG. 7 is anexplanatory view schematically illustrating each member constituting thestacked electrode body.

As illustrated in FIGS. 6 and 7, a stacked electrode body 20A isconfigured such that a plurality of rectangular positive electrodesheets 50 and a plurality of rectangular negative electrode sheets 60are stacked alternately with rectangular sheet-shaped separators 70being provided therebetween. In the stacked electrode body 20A, a coreportion 22A in which the positive-electrode mixture layers 54 and thenegative-electrode mixture layers 64 are stacked, a positive-electrodeconnecting portion 24A in which the positive-electrode exposed portions52 a are stacked, and a negative-electrode connecting portion 26A inwhich the negative-electrode exposed portions 62 a are stacked areformed. Also in the stacked electrode body 20A, the width w2 of thenegative-electrode mixture layer 64 is set to be longer than the widthw1 of the positive-electrode mixture layer 54, for example. Hereby,mixture layer non-facing portions 27A, 28A can be formed each in aboundary between the core portion 22A and a corresponding terminalconnecting portion (the positive-electrode connecting portion 24A, thenegative-electrode connecting portion 26A). Then, the short-circuitpromoting portion 72 is formed in the separator 70 in the mixture layernon-facing portion 27A where the negative-electrode mixture layer 64faces the positive-electrode exposed portion 52 a. Hereby, at the timewhen a temperature largely increases due to an excessive charge current,internal short-circuit is caused between the negative-electrode mixturelayer 64 and the positive-electrode exposed portion 52 a, so thatcharging can be stopped. On this account, with the technology describedherein, even in a case of the secondary battery including the stackedelectrode body 20A, it is possible to appropriately prevent a suddentemperature increase to a high-temperature range to be caused due tointernal short-circuit in the core portion 22.

Further, another example of the structure of the electrode body includesa cylindrical wound electrode body or the like. Even in a case wheresuch a cylindrical wound electrode body is used, when the mixture layernon-facing portion where the electrode mixture layer faces the currentcollector foil exposed portion is formed, it is possible toappropriately restrain a sudden temperature increase to ahigh-temperature range to be caused due to internal short-circuit in thecore portion 22, by forming the short-circuit promoting portion in theseparator in the mixture layer non-facing portion.

Test Examples

The following describes tests related to the present disclosure.However, the following description is not intended to limit the presentdisclosure.

A. First Test

1. Manufacture of Sample

(1) Sample 1

In Sample 1, positive slurry in which a positive-electrode activematerial (LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂), a conductive material (AB), anda binder (PVdF) were dispersed in a solvent (NMP) at a mass ratio of94:3:3 was first prepared. The positive slurry was then applied to bothsurfaces of a positive-electrode current collector foil (made ofaluminum) and dried, and thus, a positive electrode sheet wasmanufactured. Subsequently, negative slurry in which anegative-electrode active material (graphite), a thickener (CMC), and abinder (SBR) were dispersed in a solvent (water) at a mass ratio of98:1:1 was prepared. The negative slurry was then applied to bothsurfaces of a negative-electrode current collector foil (made of copper)and dried, and thus, a negative electrode sheet was manufactured. Then,HRL slurry in which inorganic particles (boehmite) and a binder (PVdF)were dispersed in a solvent (NMP) at a mass ratio of 98:2 was prepared.The HRL slurry was applied to one surface of a three-layer resin film(PP-PE-PP) having a thickness of 20μm and then dried, so that anHRL-layer applied separator was manufactured.

Subsequently, a laminated body was manufactured by laminatingsheet-shaped materials in order of the separator, the negativeelectrode, the separator, and the positive electrode such that the HRLlayer of the separator faces the negative electrode sheet. At this time,the sheet-shaped materials were laminated such that a positive-electrodeexposed portion protruded from a first side edge portion in the widthdirection, and a negative-electrode exposed portion protruded from asecond side edge portion in the width direction. Further, in the presenttest, respective laminated positions of the sheets were adjusted suchthat a mixture layer non-facing portion where a negative-electrodemixture layer faced the positive-electrode exposed portion was formedbetween a positive-electrode connecting portion and a core portion.Then, a wound body obtained by winding the laminated body was flattenedby pressing, so that a flat-shaped wound electrode body wasmanufactured.

Subsequently, a positive terminal (made of aluminum) was connected tothe positive-electrode connecting portion of the wound electrode body byultrasonic welding, and after that, a negative terminal (made of copper)was connected to a negative-electrode connecting portion by resistancewelding. Then, the wound electrode body and a nonaqueous electrolytewere accommodated in a case (made of aluminum alloy), and the case wassealed. Thus, a lithium-ion secondary battery (Sample 1) for test wasmanufactured. Note that, in the present test, as the nonaqueouselectrolyte, an electrolyte in which lithium salt (LiPF₆) was solved ata concentration of 1 mol/L in a solvent in which EC, EMC, and DMC weremixed at a volume ratio of 1:1:1 was used.

(2) Sample 2

In Sample 2, a test battery was manufactured under the same condition asSample 1, except that a recessed portion (a short-circuit promotingportion) having a depth of 10% of the thickness of the separator (thethickness of the three-layer resin film) except the thickness of the HRLlayer was formed on a surface of the separator on a side opposite to aside where the HRL layer was formed. Note that, in this sample,respective laminated positions of sheet-shaped members were adjustedsuch that the short-circuit promoting portion was placed in the mixturelayer non-facing portion where the negative-electrode mixture layerfaced the positive-electrode exposed portion. Further, in the presentsample, the position of the short-circuit promoting portion was adjustedsuch that the short-circuit promoting portion was placed on theoutermost side, in the thickness direction, of an R portion of theelectrode body after winding.

(3) Samples 3 to 5

In Samples 3 to 5, test batteries were manufactured under the samecondition as Sample 2 except that the depth of the short-circuitpromoting portion was changed. Respective depths of the short-circuitpromoting portions in Samples 3 to 5 are shown in Table 1 (describedlater).

2. Evaluation Test

In the present test, an overcharge test was performed such that athermoelectric couple was inserted into the case of each sample. Notethat the thermoelectric couple was placed in the vicinity of the coreportion in the case. In the overcharge test, a test battery was set inan environment of −10° C., and an upper limit voltage was set to 25 V.Then, constant current charging (CC charging) was performed at acharging rate of 10 C from a state where a state of charge (SOC) was10%. Then, a temperature inside the case during charging was measured tofind a maximum temperature (° C.) and find a timing when charging wasstopped by internal short-circuit. The results are shown in Table 1.

TABLE 1 1 2 3 4 5 Depth of short- — 10% 20% 25% 30% circuit promotingportion (vs separator thickness) Charging No After After After Afterstop stop six five four two timing sec sec sec sec Maximum 200° C. 200°C. 200° C. 200° C. 110° C. temperature or or or or (° C.) more more moremore

As illustrated in Table 1, in Sample 5, a sudden temperature increase toa high-temperature range was remarkedly restrained in comparison withother samples. Further, the timing when charging was stopped by internalshort-circuit was also earlier than other samples. From this point, thefollowing fact is found. That is, when the recessed portion (theshort-circuit promoting portion) having a depth of 30% or more of thethickness of the separator is formed in the separator, melting of theseparator can be promoted from a desired position. Accordingly, when theshort-circuit promoting portion is placed in the mixture layernon-facing portion, it is possible to prevent short-circuit between theelectrode mixture layers, thereby making it possible to restrain anexcessive temperature increase.

B. Second Test

1. Manufacture of Sample

Five types of test batteries (Samples 6 to 10) were manufactured underthe same condition as Sample 5 in the first test, except that theformation position of the short-circuit promoting portion in theseparator was changed. Respective formation positions of theshort-circuit promoting portions in the samples are shown in Table 2.

2. Evaluation Test

An overcharge test was performed under the same condition as the firsttest to find a maximum temperature (° C.) during charging and a chargingstop timing. The results are shown in Table 2. Note that Table 2 alsoshows the results of Samples 1, 5 in the first test for comparison.

TABLE 2 1 5 6 7 8 9 10 Short- Depth (vs — 30% 30% 30% 30% 30% 30%circuit separator promoting thickness) portion Formation OutermostOutermost Intermediate Intermediate Innermost Innermost positionperiphery - periphery - periphery - periphery - periphery-R periphery- Rportion flat portion R portion flat portion portion flat portionCharging stop timing No After two After four After four After four Aftertwo After two stop sec sec sec sec sec sec Maximum temperature 200° C.110° C. 130° C. 140° C. 145° C. 100° C. 115° C. (° C.) or more

As illustrated in Table 2, in any of Samples 5 to 10, a suddentemperature increase to a high-temperature range was restrained.Further, as a result of comparison between Samples 5, 7, 9, theformation position of the short-circuit promoting portion in theelectrode body in the thickness direction exhibited such a tendency thata temperature increase inhibiting effect becomes larger in order of theintermediate part, the outermost periphery, and the innermost periphery.Further, when Samples 5, 7, 9 in which the short-circuit promotingportion was formed in the R portion of the wound electrode body werecompared with Samples 6, 8, 10 in which the short-circuit promotingportion was formed in the flat portion, it was found that an excellenttemperature increase inhibiting effect was achieved when theshort-circuit promoting portion was formed in the R portion.

The test examples have been described as specific examples of thepresent disclosure. However, those examples are for illustration onlyand do not limit the present disclosure. The present disclosure includesthe foregoing specific examples with various modifications and changes.

What is claimed is:
 1. A nonaqueous electrolyte secondary batterycomprising: an electrode body in which a plurality of electrode sheetsis laminated via a separator; and a pair of electrode terminalselectrically connected to the electrode body, wherein: each of theelectrode sheets is a sheet-shaped electrode in which an electrodemixture layer is formed on a surface of a current collector foil, theeach of the electrode sheets including a current collector foil exposedportion in which the current collector foil is exposed, the currentcollector foil exposed portion being formed in a corresponding one ofside edge portions, in a width direction, of the each of the electrodesheets; the electrode body includes a core portion where the electrodemixture layers of the electrode sheets are laminated in a central partof the electrode body in the width direction, terminal connectingportions to which the electrode terminals are connected, respectively,the terminal connecting portions being formed in both side edge portionsof the electrode body in the width direction such that respectivecurrent collector foil exposed portions are laminated, and a mixturelayer non-facing portion formed in at least one of boundaries of thecore portion with the terminal connecting portions, the mixture layernon-facing portion being configured such that the electrode mixturelayer faces the current collector foil exposed portion; and ashort-circuit promoting portion constituted by a recessed portion havinga depth of 30% or more of a thickness of the separator is formed in theseparator provided between the electrode sheets in the mixture layernon-facing portion.
 2. The nonaqueous electrolyte secondary batteryaccording to claim 1, wherein the short-circuit promoting portion is anelongated groove extending along a side edge of the core portion.
 3. Thenonaqueous electrolyte secondary battery according to claim 1, wherein:joining portions where the terminal connecting portions are joined tothe electrode terminals, respectively, are formed in respectiveconnection parts of the terminal connecting portions with the electrodeterminals; and a total area of the short-circuit promoting portion is25% or more of an area of a joining portion closer to the short-circuitpromoting portion out of the joining portions.
 4. The nonaqueouselectrolyte secondary battery according to claim 1, wherein theshort-circuit promoting portion is formed in the separator placed on anoutermost side in a thickness direction of the electrode body.
 5. Thenonaqueous electrolyte secondary battery according to claim 1, whereinthe short-circuit promoting portion is formed in the separator placed onan inner side in a thickness direction of the electrode body.